Figure 2: Model of the origins of sDscamα and sDscamβ.
From: A large family of Dscam genes with tandemly arrayed 5′ cassettes in Chelicerata
Symbols used are the same as in Fig. 1. The sDscam gene may have originated from the sequential shortening and expansion of the Ig and FNIII domains of canonical Dscam. First, the ancestral Dscam gene underwent sequential shortening of the Ig and FNIII domains of canonical Dscam (marked by the red dashed box). Eventually, a shortened Dscam evolved in the ancestral gene. This sDscam ancestor was followed later by 5′ segmental duplication to create two or more tandemly arrayed cassettes. The duplication unit may have included both exons 1–2 encoding an Ig domain or exons 1–4 encoding 2 Ig domains and their promoters (green or blue dashed boxes). Thus, various isoforms with diverse Ig1 (sDscamα) and Ig1–2 (sDscamβ) were generated by combining alternative promoters with alternative splicing.
